Structural dynamics and resonance mitigation in upscaled floating offshore wind turbines: From 15 MW to 22 MW

To reduce the cost per installed kW, scaling up floating offshore wind turbines (FOWTs) is an inevitable trend. However, the heavy weight of tall towers restricts the growth of tower stiffness, making mainstream rigid tower designs difficult to achieve. In addition, larger structures mean larger loads, and the effects of scaling up on various wind turbine structures are not well understood. This paper aims to identify the effects of scaling up on the wind turbine tower, floater, and mooring system through a dynamic perspective by comparing the latest 22 MW wind turbine model with the 15 MW model. The comparison reveals that (a) the first fore-aft tower bending frequency of the 22 MW FOWT approaches 3P, and this mode also affects the mooring forces, and (b) the heave motion of the 22 MW turbine is excessive under extreme sea conditions. The analysis identifies two resonance phenomena: pitch motion enhances the tower frequency, causing resonance between the first fore-aft tower bending mode and the blade’s 3P frequency, and resonance between floater heave and wave loads in extreme sea conditions. Subsequently, adjustments are made to the mass distribution of the floater and the cross-sectional configuration of the floater pontoon based on the observed resonance mechanism. The new model is validated through simulation analysis of wind turbine behavior under extreme sea conditions. Compared to the original 22 MW model, the newly proposed model effectively avoids resonance frequencies. It has two additional capabilities: (a) increased pitch inertia about the center of gravity reduces the first fore-aft tower bending frequency, and (b) added mass and damping in the heave direction decrease the frequency and amplitude of heave motion.